Paul Easter and colleagues developed and tested a method to measure the collapse time of post-merger remnants. They showed that in order to detect collapse times for a GW170817-like event, a network of Einstein telescope with cosmic explorer is required. Their findings recently appeared in *Arxiv*.

It is important to measure the collapse time of a binary neutron star merger remnant, as it can help us to investigate the physics of extreme matter at high temperature and densities and improve our understanding towards short gamma-ray bursts and associated kilonova. However, due to lack of sensitivity of current detectors, we weren’t able to detect collapse time of post-merger remnants for GW170817.

Thus, Paul Easter and colleagues now developed and tested a method to measure the collapse time of post-merger remnants.

“Using Bayesian inference, we inject numerical-relativity gravitational waveforms that are forced to collapse into different interferometer configurations to measure the maximum distance at which we can recover the collapse time.”— wrote authors of the study

They showed that, in order to measure the collapse times of a post-merger remnant in a GW170817-like event at ∼40 Mpc, we require interferometer configurations of either Einstein Telescope (ET), or Einstein Telescope with Cosmic Explore (ET with CE).

They also suggested that, when Cosmic Explorer and Einstein Telescope are both operating we may detect post-merger collapse times of ∼ 10 ms. If only Einstein Telescope is fully operating then we may potentially measure post-merger collapse times of ∼10 ms except for soft equations of state like SLy.

In addition, it has been found that the stiffer equation of state, LS220, has more energy in the post-merger gravitational wave at larger times after the merger. This leads to larger maximum detection distances for LS220 equations of state relative to SLy injections for tcol ∼ 15 ms.

Morever, they suggested that, if a GW170817-like event occurred near an optimal sky position there would be a 60% increase in the detection distance. In this case, collision times of 10 ms may be detectable for ET, and ET with CE, for both equations of state, LS220 & SLy. In addition, it has been suggested that, collision times of ~ 10 ms may also be detectable for a two-detector network at A+ design sensitivity, if we can detune the proposed NEMO high frequency detector to increase sensitivity in the post-merger frequency band. This could potentially increase the sensitivity of the NEMO detector by a factor of ∼1.6 which would be enough to allow the NEMO detector with 2A+ to detect a GW170817-like post-merger collapse for collision time 10 ms.

Finally, they found that there are three predominant regions for detecting the collapse time. The first region, with small collapse times, is mainly limited by the Bayes Factor for the ratio of post-merger signal to noise. For large collapse times, waveform systematics limit detections, specifically the inability of the model to track the phase of the gravitational-wave strain. Between these two regions the signal-to-noise ratio is the limiting factor.

**Reference***: Paul J. Easter, Paul D. Lasky, Andrew R. Casey, “Can we measure the collapse time of a post-merger remnant for a future GW170817-like event?”, Arxiv, pp. 1-9, 2021. https://arxiv.org/abs/2106.04064*

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